If you ever find yourself in a situation where the other party you are trying to negotiate with is angry, I suggest you do your hardest to step aside. Lock your ego in a cage and step aside. There is no point in continuing further. It will only make the said person more resolute in their position and that is not what you want happen in a negotiation. Instead come back at another time. However, if it is a pressing issue where time is of the essence, your number one priority is to change the state of the person. Facilitate him out of his anger. Shift gears. Acknowledge his anger. Hear him out. Be empathetic. When he has let out whatever he needs to, only then can you access if it is still worth a try or to skip it altogether.